Table 2.

Hemodynamic Adaptations of Pregnancy^7–8

Part A: changes during gestation

Cardiac output increases by 30% to 50% by end of first trimester and continues its increase until the second and third trimesters.

Stroke volume is augmented particularly in early pregnancy which can result in a systolic flow murmur usually of an intensity of less than grade II/VI.

Circulating blood volume reaches a maximum of 40% above baseline at 24 weeks gestation.

Heart rate rises by 10 to 30 beats/minute late in pregnancy.

Up to 30% of women develop reversible chamber enlargement.

Preload reduction may occur due to diminished venous return related to a mechanical compression of the inferior vena cava by the gravid uterus.

Decreased systemic vascular resistance is related to the presence of placental circulation as well as the effect of vasoactive circulating substances during pregnancy.

Part B: changes during labor & delivery

Cardiac output increases by another 15% in early labor, by 25% during stage 1, and by 50% during expulsive efforts. The changes may be partially avoided by the use of epidural anesthesia.

Pain and anxiety can induce further increase in heart rate and blood pressure.

Acute increase in preload is attributed to an increase in blood volume induced by the boosting effect of uterine contraction as well as autotransfusion associated with uterine involution and resorption of leg edema from the removal of vena cava compression.

The increase in circulating blood volume is partially offset by the blood loss normally associated with delivery.